Apparatus of this type is known that includes an air blower unit, which is supplied with air from outside the vehicle through a fresh air admission duct, and which is further supplied with air drawn from inside the cabin of the vehicle so that this latter air is recycled through the apparatus. Such known apparatus also includes adjusting means for adjusting the relative flows of air through the two inlet ducts. It further includes air filtering means. The adjusting means usually comprise a valve, or a plurality of valves, which are arranged to assume, selectively, either a first or "fresh air" position or a "recycle" position. In the fresh air position, the recycling air admission duct is closed off, so that only fresh air from outside the vehicle is able to be passed to the blower unit. In the recycle position, the reverse is true, the fresh air admission duct being closed off so that the only air reaching the blower unit is that drawn from inside the vehicle. The adjusting means are usually arranged to be able to adopt various intermediate positions so that fresh air and recycling air can be mixed.
In normal operation of the vehicle, the user adjusts the adjusting means to the "fresh air" position, so that only fresh air from outside the vehicle is admitted into the cabin. It is only under exceptional circumstances, that is to say when the vehicle is travelling through a zone in which the air is particularly polluted, that the adjusting means are put into the "recycle" position so that fresh air from outside the vehicle is prevented from entering the cabin.
It is already known, from the specification of European published patent application No. EP 380453A, to provide an apparatus of the above type which further includes a single filter arranged at the confluence of the fresh air admission duct and the recycling air admission duct. This filter is designed simply for filtering the fresh air from outside. In practice, it is in the form of a particle filter, i.e. a filter which is adapted to trap particles of different kinds which are in suspension in the atmosphere outside the vehicle; air that is recycled wholly within the vehicle is not subjected to such a filtration process.
Other apparatuses are known which have a general structure similar to that in the European patent specification mentioned above, but which also include a single filter arranged at the confluence of the two air admission ducts for the purpose of filtering both the fresh air from outside the vehicle and the air being recycled within the vehicle. In this case, this single filter is a filter of a deodorant type which contains an absorbent material of the active carbon type, for absorbing or purging the odours which are carried by the air passing through the filter. Given that this deodorant filter acts indiscriminately on both the fresh air and the internal air being recycled, it is in permanent operation, and thus tends to become very rapidly dirty and saturated. Since this filter is working essentially in the "fresh air" position, when the apparatus is adjusted into the "recycle" position the filter is accordingly no longer particularly effective for the purpose of treating the air being recycled within the vehicle.
In addition, the filtering arrangements of known types mentioned above have the disadvantage of being expensive, and, more importantly, they are unable to treat effectively and permanently all of the air flow that passes through the heating and/or air conditioning installation.